Net energy effects of dietary fat on chemically induced mammary carcinogenesis in F344 rats.

The effect of net energy, as distinct from kilocalorie intake or the percent of fat in the diet, on 7,12-dimethylbenz[a]anthracene [(DMBA) CAS: 57-97-6]-induced mammary tumorigenesis in female inbred F344 rats was investigated. Rats were fed a 5% corn oil diet from weaning until DMBA administration, when they were switched to one of three dietary regimens: 5% corn oil diet, low-fat diet fed ad libitum (LF); 30% corn oil diet, high-fat diet fed ad libitum (HF); or 30% corn oil diet fed at a level providing a calculated net energy equivalent to the group on LF [high-fat diet fed at a restricted level (HF-R)]. Calculated relative net energy values of the amounts of diet actually consumed by the groups on HF-R, LF, and HF were, respectively, 0.90, 1.00, and 1.07 (kcal equivalent to 34.1, 42.2, and 40.8, respectively). Weight gain for the groups on LF and HF-R was the same throughout the experiment (24 wk), while rats on HF weighed significantly more at 6 weeks and thereafter. Body composition analyses at 24 weeks established that the groups on HF and HF-R were equivalent in fat: protein ratio, whereas the group on LF had about 35% less body fat and 15% more body protein. Carcass energy was in the following order for rats in these diet groups: HF greater than HF-R greater than LF. At 24 weeks, tumor incidences for the groups on HF, LF, and HF-R were, respectively, 73, 43, and 7%. These data indicated that tumor appearance does not depend on the percent fat in the diet per se but rather on a complex interaction involving energy intake, energy retention, and body size.